Exhaust hood and steam turbine

ABSTRACT

An exhaust hood is provided with an inner casing, an outer casing, and a diffuser. The inner casing surrounds a rotor from the outside in a radial direction, and forms a first space in which a fluid flows in an axial direction between the rotor and the inner casing. The diffuser is provided with a bearing cone that has a diameter that gradually widens moving towards an axial downstream side and forms a cylindrical shape extending to the axial downstream side to be continuous with the outer circumferential surface of a rotor shaft that forms the first space. An end edge on the axial downstream side of the bearing cone forms an oval shape in which a distance between the axial line and a second cone end part of a second side is greater than a distance between the axial line and a first cone end part.

TECHNICAL FIELD

The present invention relates to an exhaust hood and a steam turbine.

This application claims the priority of Japanese Patent Application No.2017-253815 filed in Japan on Dec. 28, 2017, the content of which isincorporated herein by reference.

BACKGROUND ART

There are many cases where rotary machines such as turbines orcompressors include a diffuser downstream of the last rotor blade forrecovering the pressure of the working fluid. In such a diffuser, theworking fluid exhausted along an axis of a rotor shaft is formed tochange a direction toward an outside in a radial direction about therotor shaft, for example, due to the layout. There is a case where sucha diffuser has a large exhaust loss due to a change in an exhaustdirection.

Patent Literatures 1 and 2 suggest a technology that forms a bearingcone shape of the diffuser into an asymmetric form between an exhaustside and an opposite exhaust side of an outer casing in order to reducethe exhaust loss from the last rotor blade of a steam turbine to acondenser.

Patent Literature 3 suggests a technology in which a flow guide of adiffuser is formed asymmetrically between an exhaust side and anopposite exhaust side of an outer casing.

CITATION LIST Patent Literature

[PTL 1] Japanese Unexamined Patent Application Publication No.2006-083801

[PTL 2] Japanese Unexamined Patent Application Publication No.2004-150357

[PTL 3] Japanese Unexamined Patent Application Publication No. 11-200814

SUMMARY OF INVENTION Technical Problem

However, even when the exhaust loss is reduced as in Patent Literatures1 to 3, a backflow occurs in the vicinity of the bearing cone on theopposite exhaust side, or delamination occurs in the flow guide on theexhaust side, there is a possibility that a pressure loss occurs.

The present invention has been made in view of the above-describedcircumstances, and provides an exhaust hood and a steam turbine that canreduce the pressure loss and improve the performance.

Solution to Problem

The following configuration is adopted to solve the above-describedproblem.

According to a first aspect of the present invention, the exhaust hoodincludes an inner casing, an outer casing, and a diffuser. The innercasing surrounds a rotor from an outside in a radial direction about anaxis of a rotor shaft, and forms a first space in which a fluid flows ina direction in which the axis extends between the rotor and the innercasing. The outer casing surrounds the rotor and the inner casing, formsa second space to which the fluid flowing through the first space isdischarged between the inner casing and the outer casing, and has anoutlet on a first side in a direction orthogonal to the axis. Thediffuser disposed on a downstream side of the inner casing to form adiffuser space communicating with the first space, is oriented radiallyoutward as going toward the downstream side, and allows the first spaceand the second space to communicate with each other. The diffuser isprovided with a bearing cone that forms a cylindrical shape extending toa downstream side in an axial direction to be continuous with an outerperipheral surface of the rotor shaft that forms the first space and hasa diameter that gradually widens as going toward the downstream side inthe axial direction. An end edge on the downstream side of the bearingcone forms an oval shape in which, in a direction orthogonal to theaxial line, a distance between the axial line and a second cone endportion on a second side opposite to the first side is greater than adistance between the axial line and a first cone end portion on thefirst side.

At the end edge on the downstream side of the bearing cone according tothe first aspect, in the direction orthogonal to the axial line, thedistance between the axial line and the second cone end portion on thesecond side opposite to the first side is greater than the distancebetween the axial line and the first cone end portion on the first side.Accordingly, for example, in a case where the first cone end portion andthe second cone end portion are at the same position in the axialdirection, or in a case where the second cone end portion is disposed onthe upstream side in the axial direction from the first cone endportion, an angle of the bearing cone with respect to the axis on thesecond side is greater than that on the first side. Therefore, thebearing cone can be formed to follow the flow of the fluid in thediffuser space on the second side. Meanwhile, in a case where the secondcone end portion is positioned on the downstream side in the axialdirection from the first cone end portion, the length of the diffuserspace on the second side can be increased. Therefore, a region where abackflow occurs can be eliminated. Accordingly, the performance can beimproved by reducing the pressure loss.

According to a second aspect of the present invention, the diffuseraccording to the first aspect may include a flow guide that forms acylindrical shape extending to the downstream side in the axialdirection from an end edge on the downstream side of the inner casingand has a diameter that gradually widens as going toward the downstreamside in the axial direction. The flow guide may include a first guidesection formed closer to the first side than the axis, and a secondguide section formed closer to the second side than the axis. In across-sectional view including the axis, a radial distance between theaxis and a second side guide end portion positioned on the most secondside of the second guide section may be greater than a radial distancebetween the axis and the first guide section which is at the sameposition as the second side guide end portion in the axial direction. Anangle between the axis and a tangent at the second side guide endportion may be greater than an angle between the axis and a tangent ofthe first guide section which is at the same position as the second sideguide end portion in the axial direction.

With this configuration, it is possible to suppress a decrease in a flowpath cross-sectional area on the second side in the diffuser space to besmaller than a flow path cross-sectional area on the first side.Therefore, it is possible to expand the effective flow path area as adiffuser space at the outlet of the diffuser, and to improve thepressure recovery performance of the diffuser.

According to a third aspect of the present invention, the diffuseraccording to the first aspect may include a flow guide that forms acylindrical shape extending to the downstream side in the axialdirection from an end edge on the downstream side of the inner casingand has a diameter that gradually widens as going toward the downstreamside in the axial direction. The flow guide may include a first guidesection formed closer to the first side than the axis, and a secondguide section formed closer to the second side than the axis. In across-sectional view including the axis, an angle between the axis and atangent of the second guide section is greater than an angle between theaxis and a tangent of the first guide section which is at the sameposition as the second guide section in the axial direction.

With this configuration, it is possible to suppress a decrease in theflow path cross-sectional area on the second side in the diffuser spaceto be smaller than the flow path cross-sectional area on the first side.Therefore, it is possible to expand the effective flow path area as adiffuser space at the outlet of the diffuser, and to improve thepressure recovery performance of the diffuser.

According to a fourth aspect of the present invention, the first coneend portion on the first side according to the second or third aspectmay be positioned on the downstream side in the axial direction from thesecond cone end portion on the second side.

In the diffuser space on the first side, the flow of the fluiddischarged from the first space inside the inner casing is oriented inthe axial direction, and thus, the bearing cone side is unlikely to bedelaminated. Therefore, by positioning the first cone end portion on thefirst side on the downstream side in the axial direction from the secondcone end portion on the second side, an effective flow pathcross-sectional area as a diffuser space on the first side can beexpanded.

According to a fifth aspect of the present invention, the diffuseraccording to the first aspect may include a flow guide that forms acylindrical shape extending to the downstream side in the axialdirection from an end edge on the downstream side of the inner casingand has a diameter that gradually widens as going toward the downstreamside in the axial direction and. The flow guide may include a firstguide section formed closer to the first side than the axis, and asecond guide section formed closer to the second side than the axis. Thesecond cone end portion may be disposed on the downstream side in theaxial direction from the first cone end portion. Furthermore, the secondguide section according to the first aspect may have a longer length inthe axial direction than that of the first guide section.

With this configuration, since the length of the bearing cone and thelength of the flow guide on the second side can be respectivelyincreased, the length of the diffuser space on the second side can beincreased. Therefore, it is possible to suppress the occurrence of thebackflow on the bearing cone side, and to improve the pressure recoveryperformance of the diffuser.

According to a sixth aspect of the present invention, in the bearingcone according to the first aspect, an end portion having a largestdistance from the axis is disposed at a position shifted forward in arotational direction of the rotor shaft from a position on the mostsecond side in a circumferential direction with the axis as the center.

With this configuration, for example, in a case where a flow including aturning component flows into the diffuser from the last rotor blade ofthe rotor, in the bearing cone, the end portion having the largestdistance from the axis can be disposed at a position at which a backflowregion is most likely to be generated. Therefore, the pressure loss inthe diffuser can be effectively reduced.

According to a seventh aspect of the present invention, a steam turbineincludes the exhaust hood according to any one of the first to sixthaspects.

With this configuration, the efficiency of the steam turbine can beimproved.

Advantageous Effects of Invention

According to the above-described exhaust hood and the steam turbine, theperformance can be improved by reducing the pressure loss.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating a schematic configuration of a steamturbine according to a first embodiment of the present invention.

FIG. 2 is an enlarged view of an exhaust hood according to the firstembodiment of the present invention.

FIG. 3 is a view illustrating an outer shape of a bearing cone and aflow guide when viewed from an axial direction in the first embodimentof the present invention.

FIG. 4 is a view corresponding to FIG. 2 in a second embodiment of thepresent invention.

FIG. 5 is a view corresponding to FIG. 3 in the second embodiment of thepresent invention.

FIG. 6 is a view corresponding to FIG. 2 in a first modification exampleof the second embodiment of the present invention.

FIG. 7 is a view corresponding to FIG. 3 in the first modificationexample of the second embodiment of the present invention.

FIG. 8 is a view corresponding to FIG. 2 in a second modificationexample of the second embodiment of the present invention.

FIG. 9 is a view corresponding to FIG. 3 in the second modificationexample of the second embodiment of the present invention.

FIG. 10 is a view corresponding to FIG. 2 in a third embodiment of thepresent invention.

FIG. 11 is a view corresponding to FIG. 3 in the third embodiment of thepresent invention.

FIG. 12 is a view corresponding to FIG. 2 in a fourth embodiment of thepresent invention.

FIG. 13 is a view corresponding to FIG. 3 in the fourth embodiment ofthe present invention.

DESCRIPTION OF EMBODIMENTS

Next, an exhaust hood and a steam turbine according to embodiments ofthe present invention will be described with reference to the drawings.

First Embodiment

FIG. 1 is a view illustrating a schematic configuration of a steamturbine according to a first embodiment of the present invention.

As illustrated in FIG. 1, a steam turbine ST of the first embodiment isa two-way exhaust type steam turbine. The steam turbine ST includes afirst steam turbine section 10 a and a second steam turbine section 10b. Each of the first steam turbine section 10 a and the second steamturbine section 10 b has a turbine rotor (rotor) 11 that rotates aboutan axis Ar, a casing 20 that covers the turbine rotor 11, and aplurality of stator blade rows 17 fixed to the casing 20, and a steaminlet duct 19. In the following description, a circumferential directionabout the axis Ar is simply referred to as a circumferential directionDc, and a direction perpendicular to the axis Ar is referred to as aradial direction Dr. Furthermore, a side toward the axis Ar in theradial direction Dr is defined as a radial inner side Dri, and a sideopposite thereto is defined as a radial outer side Dro.

The first steam turbine section 10 a and the second steam turbinesection 10 b share the steam inlet duct 19. Except for the steam inletduct 19, the first steam turbine section 10 a is disposed on one side inthe axial direction Da with the steam inlet duct 19 as a reference.Except for the steam inlet duct 19, the second steam turbine section 10b is disposed on the other side in the axial direction Da with the steaminlet duct 19 as a reference. Here, the configuration of the first steamturbine section 10 a and the configuration of the second steam turbinesection 10 b are basically the same. Therefore, in the followingdescription, the first steam turbine section 10 a will be mainlydescribed, and the description of the second steam turbine section 10 bwill be omitted. In the first steam turbine section 10 a, the side ofthe steam inlet duct 19 in the axial direction Da is defined as an axialupstream side Dau, and a side opposite thereto is defined as an axialdownstream side Dad.

The turbine rotor 11 has a rotor shaft 12 extending in the axialdirection Da about the axis Ar, and a plurality of rotor blade rows 13attached to the rotor shaft 12. The turbine rotor 11 is supported by abearing 18 to be rotatable about the axis Ar. The plurality of rotorblade rows 13 are arranged in the axial direction Da. Each of theplurality of rotor blade rows 13 is configured with a plurality of rotorblades arranged in the circumferential direction Dc. The turbine rotor11 of the first steam turbine section 10 a and the turbine rotor 11 ofthe second steam turbine section 10 b are positioned on the same axis Arand connected to each other, and rotate integrally about the axis Ar.

The casing 20 has an inner casing 21 and an exhaust casing 25.

The inner casing 21 forms a first space 21 s that forms an annular shapeabout the axis Ar, between the rotor shaft 12 and the inner casing 21.The steam (fluid) flowing from the steam inlet duct 19 flows through thefirst space 21 s in the axial direction Da (more specifically, towardthe axial downstream side Dad). The plurality of rotor blade rows 13 ofthe turbine rotor 11 are arranged in the first space 21 s. The pluralityof stator blade rows 17 are arranged in the first space 21 s along theaxial direction Da. Each of the plurality of stator blade rows 17 isarranged on the axial upstream side Dau of any one rotor blade row 13among the plurality of rotor blade rows 13. The plurality of statorblade rows 17 are fixed to the inner casing 21.

The exhaust casing 25 has a diffuser 26 and an outer casing 30.

The outer casing 30 surrounds the turbine rotor 11 and the inner casing21, and forms a second space 30 s to which the steam flowing through thefirst space 21 s is discharged, between the inner casing 21 and theouter casing 30. The second space 30 s communicates with the diffuser 26and expands on the outer peripheral side of the diffuser 26 in thecircumferential direction Dc. The outer casing 30 guides the steamflowing from a diffuser space 26 s into the second space 30 s, to theexhaust port 31.

The outer casing 30 has an exhaust port (outlet) 31 on a first side(lower side in FIG. 1) in a direction orthogonal to the axis Ar. Theouter casing 30 illustrated in the present embodiment is open verticallydownward. The steam turbine ST of the present embodiment is a so-calleddownward exhaust type condensing steam turbine, and a condenser (notillustrated) for returning steam to water is connected to the exhaustport 31. The outer casing 30 in the present embodiment includes adownstream end plate 32, an upstream end plate 34, and a side peripheralplate 36, respectively.

The downstream end plate 32 expands from the edge of the bearing cone 29on the radial outer side Dro to the radial outer side Dro, and definesthe edge of the second space 30 s on the axial downstream side Dad.

The upstream end plate 34 is disposed to be closer to the axial upstreamside Dau than the diffuser 26. The upstream end plate 34 expands fromthe outer peripheral surface 21 o of the inner casing 21 to the radialouter side Dro, and defines the edge of the second space 30 s on theaxial upstream side Dau.

The side peripheral plate 36 is connected to the downstream end plate 32and the upstream end plate 34, expands in the axial direction Da andexpands in the circumferential direction Dc about the axis Ar, anddefines the edge of the second space 30 s on the radial outer side Dro.

The diffuser 26 is disposed on the axial downstream side Dad of theinner casing 21, and allows the first space 21 s and the second space 30s to communicate with each other. The diffuser 26 forms the annulardiffuser space 26 s that gradually moves radially outward as goingtoward the axial downstream side Dad. The steam that has flowed out froma last rotor blade row 13 a of the turbine rotor 11 toward the axialdownstream side Dad flows into the diffuser space 26 s. Here, the lastrotor blade row 13 a is a rotor blade row 13 that is disposed on themost axial downstream side Dad among a plurality of rotor blade rows 13included in the first steam turbine section 10 a.

The diffuser 26 includes a flow guide (or a steam guide, also referredto as an outer diffuser) 27 that defines the edge of the diffuser space26 s on the radial outer side Dro, and a bearing cone (or referred to asan inner diffuser) 29 that defines the edge of the diffuser space 26 son the radial inner side Dri.

The bearing cone 29 is formed in a cylindrical shape extending to theaxial downstream side Dad to be continuous with an outer peripheralsurface 12 a of the rotor shaft 12 that forms the first space 21 s. Thebearing cone 29 has a ring-shaped cross section perpendicular to theaxis Ar, and the diameter thereof gradually widens toward the radialouter side Dro as going toward the axial downstream side Dad. An endedge 29 a of the bearing cone 29 is connected to the downstream endplate 32 of the outer casing 30.

The flow guide 27 has a cylindrical shape extending from the end edge ofthe inner casing 21 on the axial downstream side Dad as going toward theaxial downstream side Dad. The flow guide 27 has a ring-shaped crosssection perpendicular to the axis Ar, and the diameter thereof graduallywidens toward the axial downstream side Dad. The flow guide 27 in thepresent embodiment is connected to the inner casing 21.

An exhaust hood Ec in the present invention includes the inner casing21, the outer casing 30, and the diffuser 26.

FIG. 2 is an enlarged view of the exhaust hood according to the firstembodiment of the present invention.

FIG. 3 is a view illustrating an outer shape of the bearing cone and theflow guide when viewed from the axial direction in the first embodimentof the present invention.

Here, as illustrated in FIG. 2, since the exhaust port 31 is disposedonly on one side (first side) of the direction orthogonal to the axisAr, the exhaust hood Ec has an asymmetric shape in the circumferentialdirection Dc, and the pressure distribution in the circumferentialdirection occurs. Then, as illustrated in FIG. 2, on the side (secondside) opposite to the side where the exhaust port 31 is disposed, theflow of the steam discharged from the first space 21 s moves toward theradial outer side Dro, and the flow rate distribution (illustrated bythe two-dot chain line and the arrow inside the diffuser 26 in FIG. 2)is biased to the flow guide 27 side. In the following description, afirst side area on which the exhaust port 31 is installed with respectto the axis Ar, which is one side in the direction orthogonal to theaxis Ar, is referred as an exhaust side Dex, and a second side areawhich is opposite to the exhaust port 31 with respect to the axis Ar isreferred as an opposite exhaust side Den, and the side opposite to theexhaust port 31 across the axis Ar is referred to as an opposite exhaustside Dan (the same applies to the second and subsequent embodiments).

In the flow guide 27 of the first embodiment, the shape of the crosssection (hereinafter, referred to as a cross section including the axisAr) by a virtual plane including the axis Ar is formed in a curvedsurface shape protruding toward the axis Ar.

Furthermore, in the first embodiment, the surface length of the flowguide 27 in a cross section including the axis Ar of the flow guide 27is formed such that the exhaust side Dex is longer than the oppositeexhaust side Dan. Accordingly, while the angle between the axis Ar andthe tangent (illustrated by the chain line in FIG. 2) at an end edge 27a is approximately 90 degrees on the exhaust side Dex, the angle on theopposite exhaust side Dan is smaller than that on the exhaust side Dex.

The position of the end edge 27 a on the exhaust side Dex in the axialdirection Da is disposed on the axial downstream side Dad from theposition of the end edge 27 a on the opposite exhaust side Dan. At theend edges 27 a of the flow guide 27, a distance R1 ex between the axisAr and an exhaust side guide end portion 27 aa positioned on the mostexhaust side Dex is longer than a distance R1 an between the axis Ar andan opposite exhaust side guide end portion 27 ab positioned on the mostopposite exhaust side Dan.

As illustrated in FIG. 3, the end edge 27 a of the flow guide 27 in thefirst embodiment is formed in a semicircular shape in a half portion onthe opposite exhaust side Dan of the axis Ar, and in the half portion onthe exhaust side Dex of the axis Ar, the end edge 27 a is elongated tothe exhaust side Dex from the radius (the position illustrated by thetwo-dot chain line in FIG. 3) of the half circle of the half portion onthe opposite exhaust side Dan. In other words, the end edge 27 a of theflow guide 27 has an oval shape that is elongated toward the exhaustside ex from the opposite exhaust side Dan when viewed from the axialdirection Da. In addition, a case where the end edge 27 a of the flowguide 27 is formed in an oval shape and is formed asymmetrically on theexhaust side Dex and the opposite exhaust side Dan when viewed from theaxial direction Da has been described. However, the end edge 27 a of theflow guide 27 may be formed in a circular shape when viewed from theaxial direction Da. Further, the flow guide 27 may be formedsymmetrically on the exhaust side Dex and on the opposite exhaust sideDan.

As illustrated in FIG. 2, in the cross section including the axis Ar,the bearing cone 29 is formed in a curved surface shape protrudingtoward the axis Ar side. The position of the end edge 29 a of thebearing cone 29 in the axial direction Da is the same throughout theentire circumference in the circumferential direction Dc. At the endedge 29 a of the bearing cone 29 on the axial downstream side Dad, whenviewed in the axial direction Da, in the direction (that is, anorthogonal direction about the axis Ar) orthogonal to the axis Ar, adistance R2 an between the axis Ar and a second cone end portion 29 abon the opposite exhaust side Dan has a greater oval shape than that of adistance R2 ex between the axis Ar and a first cone end portion 29 aa onthe exhaust side Dex.

At the same position in the axial direction Da, the angle between theaxis Ar and the tangent (illustrated by the chain line in FIG. 2) in thevicinity of the end edge 29 b of the bearing cone 29 on the axialupstream side Dau is greater on the opposite exhaust side Dan than thaton the exhaust side Dex. Specifically, at the end edge 29 b on the axialupstream side Dau, an angle 9 e between the axis Ar and the tangent atan end portion 29 ba on the exhaust side Dex satisfies θe≥0. Inaddition, at the end edge 29 b on the axial upstream side Dau, an angleθa between the axis Ar and the tangent at an end portion 29 bb on theopposite exhaust side Dan satisfies θa≥θe≥0. Hereinafter, the anglebetween the axis Ar and the tangent is simply referred to as an angle ofthe tangent.

Regarding the angle of the tangent (illustrated by the chain line inFIG. 2) at the end edge 29 a of the bearing cone 29, an angle θoa of thetangent at the second cone end portion 29 ab on the opposite exhaustside Dan is greater than the angle θoe of the tangent at the first coneend portion 29 aa on the exhaust side Dex (θoa>θoe). In FIG. 2, theangles θoa and θoe are respectively illustrated as angles with respectto a virtual line (illustrated by the chain line in FIG. 2) parallel tothe axis Ar (the same applies to the second and subsequent embodiments).

Here, on the opposite exhaust side Dan (upper side in FIG. 2) of FIG. 2,the two-dot chain line illustrated by the axial downstream side Dad ofthe bearing cone 29 illustrates a case (comparative example) in whichthe shape of the bearing cone 29 on the exhaust side Dex is adopted atthe entire circumference in the circumferential direction Dc about theaxis Ar. In other words, in the above-described first embodiment, theposition of the bearing cone 29 on the opposite exhaust side Dan movesto the axial upstream side Dau as compared with the comparative example.

According to the above-described first embodiment, at the end edge 29 aof the bearing cone on the axial downstream side Dad, the distance R2 anbetween the axis Ar and the second cone end portion 29 ab on theopposite exhaust side Dan is greater than the distance R2 ex between theaxis Ar and the first cone end portion 29 aa on the exhaust side Dex.Accordingly, for example, in a case where the first cone end portion 29aa and the second cone end portion 29 ab are disposed at the sameposition in the axial direction Da, regarding the angle of the tangentof the bearing cone 29 with respect to the axis Ar, the angle Ga of thetangent on the opposite exhaust side Dan is greater than the angle θe ofthe tangent on the exhaust side Dex. Therefore, the bearing cone 29 canbe formed to follow the flow of the steam in the diffuser space 26 s onthe opposite exhaust side Dan side. Therefore, it is possible toeliminate the region where the backflow occurs on the opposite exhaustside Dan. As a result, the pressure loss in the diffuser 26 can bereduced and the performance can be improved.

Second Embodiment

Next, a second embodiment of the present invention will be describedwith reference to the drawings. The second embodiment is different fromthe above-described first embodiment in the shape of the flow guide onthe opposite exhaust side Dan and the shape of the flow guide on theexhaust side Dex. Therefore, the same reference numerals will be givento the same parts as those of the above-described first embodiment, andthe redundant description thereof will be omitted.

FIG. 4 is a view corresponding to FIG. 2 in the second embodiment of thepresent invention. FIG. 5 is a view corresponding to FIG. 3 in a firstmodification example of the second embodiment of the present invention.

As illustrated in FIGS. 4 and 5, similar to the above-described firstembodiment, a casing 220 of a first steam turbine section 210 a in thesecond embodiment has the inner casing 21 and an exhaust casing 225. Theexhaust casing 225 has a diffuser 226 and the outer casing 30.

The diffuser 226 is disposed on the axial downstream side Dad of theinner casing 21, and allows the first space 21 s and the second space 30s to communicate with each other. The diffuser 226 forms an annulardiffuser space 226 s that gradually moves radially outward as goingtoward the axial downstream side Dad. The steam that has flowed out fromthe last rotor blade row 13 a of the turbine rotor 11 toward the axialdownstream side Dad flows into the diffuser space 226 s.

The diffuser 226 includes a flow guide 227 that defines the edge of thediffuser space 226 s on the radial outer side Dro, and the bearing cone29 that defines the edge of the diffuser space 226 s on the radial innerside Dri. Since the bearing cone 29 has the same configuration as thatof the first embodiment, a detailed description thereof will be omitted.

The flow guide 227 has a cylindrical shape extending from the end edgeof the inner casing 21 on the axial downstream side Dad as going towardthe axial downstream side Dad. The flow guide 227 has a ring-shapedcross section perpendicular to the axis Ar, and the diameter thereofgradually widens as going toward the axial downstream side Dad. The flowguide 227 in the second embodiment is connected to the inner casing 21.Here, in order to facilitate connection (or assembly) between the flowguide 227 and the inner casing 21, there is a case where a cylindricalpart that is formed integrally with the flow guide 227 and extends inthe axial direction Da is present between the flow guide 227 and theinner casing 21. Since the cylindrical part does not function as thediffuser 226, the part is not included in the flow guide 227 (the sameapplies to each embodiment and each modification example).

In the flow guide 227 according to the second embodiment, similar to thefirst embodiment, the cross-sectional shape including the axis Ar isformed into a curved surface shape protruding toward the axis Ar.Furthermore, in the second embodiment, the surface length of the flowguide 27 in the cross section including the axis Ar of the flow guide 27is formed to be longer on the exhaust side Dex than that on the oppositeexhaust side Dan. Accordingly, while the angle of the tangent(illustrated by the chain line in FIG. 4) at an end edge 227 a isapproximately 90 degrees on the exhaust side Dex, the angle (9 sa) onthe opposite exhaust side Dan is smaller than that on the exhaust sideDex.

The flow guide 227 includes a first guide section 227A on the exhaustside Dex of the axis Ar, and includes a second guide section 227B on theopposite exhaust side Dan of the axis Ar. The first guide section 227Aand the second guide section 227B have an asymmetric shape.

The position of the end edge 227 a on the exhaust side Dex in the axialdirection Da is disposed on the axial downstream side Dad from theposition of the end edge 227 a on the opposite exhaust side Dan. At theend edge 227 a of the flow guide 27, the distance R1 ex between the axisAr and an exhaust side guide end portion 227 aa positioned on the mostexhaust side Dex is longer than the distance Rfa (=R1 an) between theaxis Ar and an opposite exhaust side guide end portion 227 ab positionedon the most opposite exhaust side Dan in the radial direction Dr.

As illustrated in FIG. 5, the end edge 227 a of the flow guide 227 inthe second embodiment is formed in an oval shape which is elongated tothe opposite exhaust side Dan or elongated to the exhaust side Dex, morethan the distance at which the distance between the axis Ar and the endedge 227 a is the shortest. The length R1 ex of the oval in the firstguide section 227A in the elongated radial direction is formed to belonger than the length R1 an of the oval in the second guide section227B in the elongated radial direction.

The distance Rfa (=R1 an) between the axis Ar and the opposite exhaustside guide end portion 227 ab positioned on the most opposite exhaustside Dan is greater than the distance Rfe between the axis Ar and thefirst guide section 227A which is at the same position as the oppositeexhaust side guide end portion 227 ab in the axial direction Da, in theradial direction Dr (Rfa>Rfe). An angle θse of the tangent in the firstguide section 227A at the same position as the opposite exhaust sideguide end portion 227 ab in the axial direction Da is smaller than theangle 9 sa of the tangent at the opposite exhaust side guide end portion227 ab (θse<θsa). In other words, the angle 9 sa of the tangent at theopposite exhaust side guide end portion 227 ab is greater than the angleθse of the tangent at the first guide section 227A at the same positionas the opposite exhaust side guide end portion 227 ab in the axialdirection Da.

Here, in FIG. 4, a comparative example in which the second guide section227B is formed at the same angle as that of the flow guide 27 of thefirst embodiment on the opposite exhaust side Dan is illustrated by thetwo-dot chain line. In other words, by forming the second guide section227B as described above, the dimension of the second guide section 227Bin the axial direction Da is shorter than the dimension of the firstguide section 227A in the axial direction Da. Furthermore, the positionof the opposite exhaust side guide end portion 227 ab of the secondguide section 227B can be disposed further on the axial upstream sideDau and on the radial outer side Dro compared to the comparativeexample. In FIG. 4, the arrangement of the flow guide 27 in theabove-described first embodiment is illustrated by the two-dot chainline on the axial upstream side Dau of the first guide section 227A.

By doing so, the angle of the tangent at the same position in the axialdirection Da in the first guide section 227A is smaller than the angle(not illustrated) of the tangent of the flow guide 227 at a boundaryposition K (refer to FIG. 5) between the first guide section 227A andthe second guide section 227B.

Therefore, according to the second embodiment, since the first guidesection 227A extends to the axial downstream side Dad and follows theflow of steam, in the diffuser space 226 s on the exhaust side Dex, theoccurrence of delamination on the first guide section 227A side can bemore suppressed than the second guide section 227B.

First Modification Example of Second Embodiment

Next, a first modification example of the second embodiment of thepresent invention will be described with reference to the drawings. Thesame reference numerals will be given to the same parts as those of theabove-described second embodiment, and the redundant description thereofwill be omitted.

FIG. 6 is a view corresponding to FIG. 2 in the first modificationexample of the second embodiment of the present invention. FIG. 7 is aview corresponding to FIG. 3 in the first modification example of thesecond embodiment of the present invention.

As illustrated in FIGS. 6 and 7, similar to the above-described secondembodiment, a casing 220X of the first steam turbine section 210 a inthe first modification example of the second embodiment has the innercasing 21 and the exhaust casing 225. The exhaust casing 225 has thediffuser 226 and the outer casing 30.

The diffuser 226 is disposed on the axial downstream side Dad of theinner casing 21, and allows the first space 21 s and the second space 30s to communicate with each other. The diffuser 226 forms an annulardiffuser space 226 s that gradually moves radially outward as goingtoward the axial downstream side Dad. The steam that has flowed out fromthe last rotor blade row 13 a of the turbine rotor 11 toward the axialdownstream side Dad flows into the diffuser space 226 s.

The diffuser 226 includes a flow guide 227 that defines the edge of thediffuser space 226 s on the radial outer side Dro, and the bearing cone29 that defines the edge of the diffuser space 226 s on the radial innerside Dri.

The flow guide 227 has a cylindrical shape extending from the end edgeof the inner casing 21 on the axial downstream side Dad as going towardthe axial downstream side Dad. The flow guide 227 has a ring-shapedcross section perpendicular to the axis Ar, and the diameter thereofgradually widens as going toward the axial downstream side Dad. The flowguide 227 in the second embodiment is connected to the inner casing 21.

In the flow guide 227 according to the first modification example of thesecond embodiment, similar to the first and second embodiments, thecross-sectional shape including the axis Ar is formed into a curvedsurface shape protruding toward the axis Ar. Furthermore, in the secondembodiment, the surface length of the flow guide 27 in the cross sectionincluding the axis Ar of the flow guide 27 is formed to be longer on theexhaust side Dex than that on the opposite exhaust side Dan.

Accordingly, while the angle of the tangent (illustrated by a chain linein FIG. 6) at the end edge 227 a is approximately 90 degrees on theexhaust side Dex, the angle on the opposite exhaust side Dan is smallerthan that on the exhaust side Dex.

The flow guide 227 includes a first guide section 227AX on the exhaustside Dex of the axis Ar, and includes the second guide section 227B onthe opposite exhaust side Dan of the axis Ar. The first guide section227AX and the second guide section 227B have an asymmetric shape.

The position of the end edge 227 a on the exhaust side Dex in the axialdirection Da is disposed on the axial downstream side Dad from theposition of the end edge 227 a on the opposite exhaust side Dan. At theend edge 227 a of the flow guide 227, the distance R1 ex between theaxis Ar and the exhaust side guide end portion 227 aa positioned on themost exhaust side Dex is longer than the distance R1 an between the axisAr and the opposite exhaust side guide end portion 227 ab positioned onthe most opposite exhaust side Dan in the radial direction Dr.

As illustrated in FIG. 7, the end edge 227 a of the flow guide 227 inthe second embodiment is formed in an oval shape which is elongated tothe opposite exhaust side Dan or elongated to the exhaust side Dex, morethan the distance at which the distance between the axis Ar and the endedge 227 a is the shortest. A length Roe of the oval in the first guidesection 227AX in the elongated radial direction is formed to be longerthan a length Roa of the oval in the second guide section 227B in theelongated radial direction.

As illustrated in FIG. 6, in the cross section including the axis Ar,the angle of the tangent (illustrated by the chain line in FIG. 6) ofthe flow guide 227 at the same position in the axial direction Da isgreater on the opposite exhaust side Dan than that on the exhaust sideDex. Specifically, at the same position in the axial direction Da, theangle θfe of the tangent of the first guide section 227AX is equal to orgreater than 0 degree and smaller than the angle θfa of the tangent ofthe second guide section 227B (θfa>θfe≥0). Here, in FIG. 6, acomparative example in which the second guide section 227B is formed atthe same angle θfe as the first guide section 227AX on the oppositeexhaust side Dan is illustrated by the two-dot chain line. In otherwords, by forming the above-described second guide section 227B, thedimension of the second guide section 227B in the axial direction Da isshorter than the dimension of the first guide section 227AX in the axialdirection Da. Furthermore, the position of the opposite exhaust sideguide end portion 227 ab of the second guide section 227B can bedisposed on the axial upstream side Dau and on the radial outer side Drocompared to the comparative example in which the angle θfe is set.

Since the bearing cone 29 has the same configuration as that of thefirst and second embodiments, a detailed description thereof will beomitted.

Therefore, according to the first modification example of theabove-described second embodiment, it is possible to suppress thedecrease in the flow path cross-sectional area of the diffuser space 226s on the opposite exhaust side Dan to be smaller than the flow pathcross-sectional area on the exhaust side Dex. Therefore, it is possibleto expand the effective flow path area as the diffuser space 226 s atthe outlet of the diffuser 226, and to improve the pressure recoveryperformance of the diffuser 226.

Second Modification Example of Second Embodiment

FIG. 8 is a view corresponding to FIG. 2 in the second modificationexample of the second embodiment of the present invention. FIG. 9 is aview corresponding to FIG. 3 in the second modification example of thesecond embodiment of the present invention.

In the first modification example of the above-described secondembodiment, the first guide section 227AX is formed to extend to theaxial downstream side Dad more than the first guide section 227A of thesecond embodiment. Similar to the flow guide 227 of the firstmodification example, for example, as in the second modification exampleillustrated in FIGS. 8 and 9, a bearing cone 229X on the exhaust sideDex may be formed to extend to the axial downstream side Dad more thanthe bearing cone 29 (illustrated by the two-dot chain line in FIG. 8) onthe exhaust side Dex of the second embodiment.

In other words, a first cone end portion 229 aa on the exhaust side Dexof the bearing cone 229X may be disposed on the axial downstream sideDau from a second cone end portion 229 ab on the opposite exhaust sideDan. In addition, a case where the position of the first cone endportion 229 aa in the radial direction Dr in the second modificationexample is the same as the position of the first cone end portion 29 aain the radial direction Dr in the first and second embodiments isillustrated, but may be closer to the axis Ar than the position.

Therefore, according to the second modification example of the secondembodiment, the effective flow path area of the diffuser 226 on theexhaust side Dex of the diffuser 226 can be expanded to the axialdownstream side Dad compared to the first modification example.Accordingly, the performance of the diffuser 26 can be improved.

Third Embodiment

Next, a third embodiment of the present invention will be described withreference to the drawings. The third embodiment is different from theabove-described second embodiment in the shapes of the flow guide andthe bearing cone on the opposite exhaust side Dan. Therefore, the samereference numerals will be given to the same parts as those of theabove-described second embodiment, and the redundant description thereofwill be omitted.

FIG. 10 is a view corresponding to FIG. 2 in the third embodiment of thepresent invention. FIG. 11 is a view corresponding to FIG. 3 in thethird embodiment of the present invention.

As illustrated in FIGS. 10 and 11, similar to the above-described secondembodiment, a casing 320 of a first steam turbine section 310 a in thethird embodiment has the inner casing 21 and an exhaust casing 325.Furthermore, the exhaust casing 325 has a diffuser 326 and the outercasing 30.

The diffuser 326 is disposed on the downstream side of the inner casing21, and allows the first space 21 s and the second space 30 s tocommunicate with each other. The diffuser 326 forms the annular diffuserspace 326 s that gradually moves radially outward as going toward theaxial downstream side Dad. The steam that has flowed out from the lastrotor blade row 13 a of the turbine rotor 11 toward the axial downstreamside Dad flows into the diffuser space 326 s.

The diffuser 326 includes a flow guide 327 that defines the edge of thediffuser space 326 s on the radial outer side Dro, and the bearing cone329 that defines the edge of the diffuser space 326 s on the radialinner side Dri.

Similar to the flow guide 227 of the second embodiment, the flow guide327 has a cylindrical shape extending from the end edge of the innercasing 21 on the axial downstream side Dad as going toward the axialdownstream side Dad. The flow guide 327 has a ring-shaped cross sectionperpendicular to the axis Ar, and the diameter thereof gradually widensas going toward the axial downstream side Dad. Similar to the secondembodiment, the flow guide 327 in the third embodiment is connected tothe inner casing 21.

In the flow guide 327 according to the third embodiment, similar to thesecond embodiment, the cross-sectional shape including the axis Ar isformed into a curved surface shape protruding toward the axis Ar.Furthermore, in the third embodiment, the arc length in a cross sectionincluding the axis Ar of the flow guide 327 is formed to be longer onthe exhaust side Dex than that on the opposite exhaust side Dan.Accordingly, while the angle of the tangent (illustrated by the chainline in FIG. 10) at an end edge 327 a is approximately 90 degrees on theexhaust side Dex, the angle on the opposite exhaust side Dan is smallerthan that on the exhaust side Dex.

The flow guide 327 includes a first guide section 327A on the exhaustside Dex of the axis Ar, and includes a second guide section 327B on theopposite exhaust side Dan of the axis Ar. The first guide section 327Aand the second guide section 327B have an asymmetric shape.

The position of the end edge 327 a on the exhaust side Dex in the axialdirection Da is disposed on the axial upstream side Dau from theposition of the end edge 327 a on the opposite exhaust side Dan. At theend edges 327 a of the flow guide 27, the distance R1 ex between theaxis Ar and an exhaust side guide end portion 327 aa positioned on themost exhaust side Dex in the radial direction Dr is longer than thedistance R1 an between the axis Ar and an opposite exhaust side guideend portion 327 ab positioned on the most opposite exhaust side Dan inthe radial direction Dr (R1 ex>R1 an).

As illustrated in FIG. 11, the end edge 327 a of the flow guide 327 inthe third embodiment is formed in a long oval shape on the exhaust sideDex and on the opposite exhaust side Dan when viewed from the axialdirection Da. The length R1 ex of the first guide section 327A in theelongated radial direction is formed to be longer than the length Rianof the second guide section 327B in the elongated radial direction (R1ex>R1 an). In other words, as illustrated in FIG. 10, the distance R1 anbetween the axis Ar and the opposite exhaust side guide end portion 327ab on the opposite exhaust side Dan is shorter than the distance R1 exbetween the axis Ar and the exhaust side guide end portion 327 aa of theflow guide 327 on the exhaust side Dex. In the cross section includingthe axis Ar illustrated in FIG. 10, the relationship between aninclination θfe of the tangent of the first guide section 327A and aninclination θfa of the tangent of the second guide section 327B at thesame position in the axial direction Da satisfies θfe≥θfa≥0.

In the cross section including the axis Ar illustrated in FIG. 10,regarding the length of the flow guide 327 in the axial direction Da, alength Lfa of the second guide section 327B is longer than a length Lfeof the first guide section 327A (Lfa>Lfe). More specifically, the lengthof the flow guide 327 in the axial direction Da is formed to graduallyincrease as going toward the opposite exhaust side Dan from the exhaustside Dex. In FIG. 10, a comparative example in which the second guidesection 327B is formed at the same angle θfe as that of the first guidesection 327A on the opposite exhaust side Dan is illustrated by thetwo-dot chain line.

In the cross section including the axis Ar, the bearing cone 329 isformed in a curved surface shape protruding toward the axis Ar side. Theend edge 329 a of the bearing cone 329 on the axial downstream side Dadis formed in an oval shape in which a distance R2 an between the axis Arand a second cone end portion 329 ab on the opposite exhaust side Dan isgreater than a distance R2 ex between the axis Ar and a first cone endportion 329 aa on the exhaust side Dex in the direction (that is, theorthogonal direction about the axis Ar) orthogonal to the axis Ar.

In the cross section including the axis Ar, at the same position in theaxial direction Da, the angle between the axis Ar and the tangent in thevicinity of the end edge 329 b of the bearing cone 329 on the axialupstream side Dau is the same on the exhaust side Dex and on theopposite exhaust side Dan. In other words, the angle θe of the tangentof the bearing cone 329 at an end portion 329 bb on the exhaust side Dexis the same as the angle θa of the tangent of the bearing cone 329 at anend portion 329 ba on the opposite exhaust side Dan. More specifically,the angles θa and θe of the tangent satisfy θa=θe≥0.

In the axial direction Da, the bearing cone 329 on the opposite exhaustside Dan extends to the axial downstream side Dad more than the bearingcone 329 on the exhaust side Dex. In other words, a length La of thebearing cone 329 on the opposite exhaust side Dan in the axial directionDa is longer than a length Le of the bearing cone 329 on the exhaustside Dex (La>Le). In addition, since the length of the bearing cone 329in the axial direction Da is different on the exhaust side Dex and onthe opposite exhaust side Dan, the angle θoa of the tangent at thesecond cone end portion 329 ab on the opposite exhaust side Dan isgreater than the angle θoe of the tangent at the first cone end portion329 aa on the exhaust side Dex (θoa>θoe).

Therefore, according to the above-described third embodiment, the lengthof the bearing cone 329 and the length of the flow guide 327 on theopposite exhaust side Dan can be respectively increased. Accordingly,the length of the diffuser space 326 s on the opposite exhaust side Dancan be increased. As a result, it is possible to suppress the occurrenceof the backflow in the flow of the steam on the bearing cone 329 side,and to improve the pressure recovery performance of the diffuser 326. Onthe other hand, since the dimension of the exhaust hood Ec in the axialdirection Da does not increase on the exhaust port 31 side where thereis a possibility that the condenser (not illustrated) or the like isdisposed, the influence on the degree of freedom of arrangement of thecondenser and the like can be suppressed.

Fourth Embodiment

Next, a fourth embodiment of the present invention will be describedwith reference to the drawings. The fourth embodiment is different fromthe above-described first embodiment in the flow guide about the axis.Therefore, the same reference numerals will be given to the same partsas those of the above-described first embodiment, and the redundantdescription thereof will be omitted.

FIG. 12 is a view corresponding to FIG. 2 in the fourth embodiment ofthe present invention. FIG. 13 is a view corresponding to FIG. 3 in thefourth embodiment of the present invention.

As illustrated in FIG. 12, similar to the above-described firstembodiment, a casing 420 of a first steam turbine section 410 a in thefourth embodiment has the inner casing 21 and an exhaust casing 425.Furthermore, the exhaust casing 425 has a diffuser 426 and the outercasing 30.

The diffuser 426 is disposed on the axial downstream side Dad of theinner casing 21, and allows the first space 21 s and the second space 30s to communicate with each other. The diffuser 426 forms the annulardiffuser space 426 s that gradually moves radially outward as goingtoward the axial downstream side Dad. The steam that has flowed out fromthe last rotor blade row 13 a of the turbine rotor 11 toward the axialdownstream side Dad flows into the diffuser space 426 s.

The diffuser 426 includes a flow guide 27 that defines the edge of thediffuser space 426 s on the radial outer side Dro, and the bearing cone429 that defines the edge of the diffuser space 426 s on the radialinner side Dri. Since the flow guide 27 has the same configuration asthe flow guide 27 of the first embodiment, the detailed descriptionthereof will be omitted.

The bearing cone 429 is different from the bearing cone 29 of the firstembodiment in the angle in the circumferential direction Dc. An end edge429 a of the bearing cone 429 on the axial downstream side Dad has anoval shape when viewed from the axial direction Da.

As illustrated in FIG. 13, at the end edge 429 a of the bearing cone429, a second cone end portion 429 ab on the opposite exhaust side Danwhere the distance from the axis Ar is the largest is disposed at aposition shifted forward in the rotational direction of the rotor shaft12 from the position (that is, a position farthest from the exhaust port31 in the circumferential direction Dc about the axis Ar) of an edgeportion 429 ac on the most opposite exhaust side Dan at the end edge 429a of the bearing cone 429. In other words, based on the position (theposition on the straight line illustrated by the one-dot chain line inFIG. 13) of the second cone end portion 29 ab of the first embodiment,the position of the second cone end portion 429 ab is shifted forward inthe rotational direction of the rotor shaft 12 in the circumferentialdirection Dc from the position of the second cone end portion 29 ab.

In other words, when viewed from the axial direction Da, with respect toa virtual line 27 f which is a straight line passing through the exhaustside guide end portion 27 aa, the opposite exhaust side guide endportion 27 ab, and the axis Ar in the flow guide 27, a virtual line 429f passing through a first cone end portion 429 aa, the second cone endportion 429 ab, and the axis Ar is disposed at a position shiftedforward in the rotational direction of the rotor shaft 12. Although thevirtual line 27 f passing through the exhaust side guide end portion 27aa and the opposite exhaust side guide end portion 27 ab in the flowguide 27 has been described as a reference position in thecircumferential direction Dc, for example, on any virtual circle (truecircle) about the axis Ar, the straight line passing through an exhaustside end portion T1 which is the most exhaust side Dex, an oppositeexhaust side end portion T2 which is the most opposite exhaust side Dan,and the axis Ar respectively may be the virtual line 27 f.

Here, an angle θr between the virtual line 27 f and the virtual line 429f is smaller than 45 degrees and greater than 0 degrees. Furthermore,the angle θr may be smaller than 30 degrees, and can be smaller than 20degrees. The angle θr may be determined, for example, in accordance witha turning component included in the flow of steam discharged from thefirst space 21 s.

Therefore, according to the fourth embodiment, in a case where a flow ofthe steam including the turning component from the last rotor blade row13 a of the turbine rotor 11 to the diffuser 426, at the bearing cone429, the second cone end portion 429 ab having the largest distance fromthe axis Ar can be disposed at a position at which a backflow region ismost likely to be generated. Therefore, the pressure loss in thediffuser 426 can be effectively reduced.

The present invention is not limited to the configuration of each of theabove-described embodiments, and the design can be changed withoutdeparting from the gist of the present invention.

For example, in each of the above-described embodiments, the exhausthood of the steam turbine has been described as an example, but thepresent invention can be applied to, for example, an exhaust hood of agas turbine or a turbomachine.

INDUSTRIAL APPLICABILITY

According to the exhaust hood and the steam turbine of the presentinvention, the performance can be improved by reducing the pressureloss.

REFERENCE SIGNS LIST

-   -   10 a, 210 a, 310 a, 410 a first steam turbine section    -   10 b second steam turbine section    -   11 rotor    -   11 turbine rotor    -   12 rotor shaft    -   12 a outer peripheral surface    -   13 rotor blade row    -   13 a last rotor blade row    -   17 stator blade row    -   18 bearing    -   19 steam inlet duct    -   20, 220, 320, 420 casing    -   21 inner casing    -   21 o outer peripheral surface    -   21 s first space    -   25, 225, 325, 425 exhaust casing    -   26, 226, 326, 426 diffuser    -   26 s, 226 s, 326 s, 426 s diffuser space    -   27, 227, 327 flow guide    -   27 a, 227 a, 327 a end edge    -   27 aa, 227 aa, 327 aa exhaust side guide end portion    -   27 ab, 227 ab, 327 ab opposite exhaust side guide end portion    -   27 f virtual line    -   29, 229, 229X, 329, 429 bearing cone    -   29 a, 329 a, 429 a end edge    -   29 aa, 229 aa, 329 aa, 429 aa first cone end portion    -   29 ab, 229 ab, 329 ab, 429 ab second cone end portion    -   29 b, 329 b end edge    -   29 ba, 329 ba end portion    -   29 bb, 329 bb end portion    -   30 outer casing    -   30 s second space    -   31 exhaust port    -   32 downstream end plate    -   34 upstream end plate    -   36 side peripheral plate    -   227A, 227AX, 327A first guide section    -   227B, 327B second guide section    -   429 ac edge portion    -   429 f virtual line    -   Ec exhaust hood    -   ST steam turbine

1-7. (canceled)
 8. An exhaust hood comprising: an inner casing thatsurrounds a rotor from an outside in a radial direction about an axis ofa rotor shaft, and forms a first space in which a fluid flows in adirection in which the axis extends between the rotor and the innercasing; an outer casing that surrounds the rotor and the inner casing,forms a second space to which the fluid flowing through the first spaceis discharged between the inner casing and the outer casing, and has anoutlet on a first side in a direction orthogonal to the axis; and adiffuser that is disposed on a downstream side of the inner casing toform a diffuser space communicating with the first space, is orientedradially outward as going toward the downstream side, and allows thefirst space and the second space to communicate with each other, whereinthe diffuser is provided with a bearing cone that forms a cylindricalshape extending to a downstream side in an axial direction to becontinuous with an outer peripheral surface of the rotor shaft thatforms the first space and has a diameter that gradually widens as goingtoward the downstream side in the axial direction, and wherein an endedge on the downstream side of the bearing cone forms an oval shape inwhich, in a direction orthogonal to the axial line, a distance betweenthe axial line and a second cone end portion on a second side oppositeto the first side is greater than a distance between the axial line anda first cone end portion on the first side, wherein in a cross-sectionalview including the axis, an angle θe formed between the axis and atangent on the side closer to the axis upstream end of the bearing coneon the exhaust side than the axis is set, an angle θa formed between theaxis and a tangent on the side closer to the axis upstream end of thebearing cone on the opposite exhaust side than the axis is set, and thebearing cone satisfies the following expression: θa>θe≥0.
 9. The exhausthood according to claim 8, wherein the diffuser includes a flow guidethat forms a cylindrical shape extending to the downstream side in theaxial direction from an end edge on the downstream side of the innercasing and has a diameter that gradually widens as going toward thedownstream side in the axial direction, wherein the flow guide includesa first guide section formed closer to the first side than the axis, anda second guide section formed closer to the second side than the axis,and wherein, in a cross-sectional view including the axis, an anglebetween the axis and a tangent of the second guide section is greaterthan an angle between the axis and a tangent of the first guide sectionwhich is at the same position as the second guide section in the axialdirection.
 10. The exhaust hood according to claim 8, wherein thediffuser includes a flow guide that forms a cylindrical shape extendingto the downstream side in the axial direction from an end edge on thedownstream side of the inner casing and has a diameter that graduallywidens as going toward the downstream side in the axial direction,wherein the flow guide includes a first guide section formed closer tothe first side than the axis, and a second guide section formed closerto the second side than the axis, wherein, in a cross-sectional viewincluding the axis, a radial distance between the axis and a second sideguide end portion positioned on the most second side of the second guidesection is greater than a radial distance between the axis and the firstguide section which is at the same position as the second side guide endportion in the axial direction, and wherein an angle between the axisand a tangent at the second side guide end portion is greater than anangle between the axis and a tangent of the first guide section which isat the same position as the second side guide end portion in the axialdirection.
 11. The exhaust hood according to claim 9, wherein the firstcone end portion on the first side is positioned on the downstream sidein the axial direction from the second cone end portion on the secondside.
 12. The exhaust hood according to claim 10, wherein the first coneend portion on the first side is positioned on the downstream side inthe axial direction from the second cone end portion on the second side.13. The exhaust hood according to claim 8, wherein, in the bearing cone,an end portion having a largest distance from the axis is disposed at aposition shifted forward in a rotational direction of the rotor shaftfrom a position on the most second side in a circumferential directionabout the axis.
 14. A steam turbine comprising: the exhaust hoodaccording to claim
 8. 15. A steam turbine comprising: the exhaust hoodaccording to claim
 9. 16. A steam turbine comprising: the exhaust hoodaccording to claim
 10. 17. A steam turbine comprising: the exhaust hoodaccording to claim
 11. 18. A steam turbine comprising: the exhaust hoodaccording to claim
 12. 19. A steam turbine comprising: the exhaust hoodaccording to claim 13.